This is a continuation of application Ser. No. 765,615, filed Aug. 13, 1985, now abandoned, which is a continuation of application Ser. No. 625,183, filed June 27, 1984, now abandoned, which is a a continuation of application Ser. No. 332,895, filed Dec. 21, 1981, also now abandoned.
The invention relates to a semiconductor device comprising at least one circuit element and having a semiconductor body. The semiconductor body comprises at least a first and a second electrode, which first electrode is connected to the said circuit element, and a safety device is provided between the said first and second electrode so as to protect the first electrode from the occurrence of excess voltages above a previously determined value with respect to the voltage of the second electrode, which safety device comprises a lateral bipolar transistor having an emitter zone and a collector zone of a first conductivity type which adjoin a surface of the semiconductor body in laterally separated locations and having an adjoining base region of a second conductivity type which forms a first and a secnd p-n junction with the said emitter zone, and collector zone respectively, an insulating layer being present on the said surface of the semiconductor body, the base region between the said emitter zone and collector zone adjoining the insulating layer, the said insulating layer separating first, second and third conductive layers from the said semiconductor body, the first electrode comprising the first conductive layer which is connected to the said collector zone, the second electrode comprising the second conductive layer which is connected to the said emitter zone and the third conductive layer extending above the said base region between the said emitter zone and collector zone, and the said second junction having a breakdown voltage which is higher than the collector-emitter voltage of the lateral bipolar transistor in the conductive state of said transistor.
A semiconductor device with a safety device comprising a lateral bipolar transistor as described above is known from British Patent Specification 1,337,220.
In this known device the third conductive layer which extends above the base region is connected to the emitter zone so as to provide a "field relief electrode" to reduce the avalanche breakdown voltage of the collector junction. For that purpose the insulating layer has a thin portion at the area where the third conductive layer extends above the collector-base junction.
Since modern semiconductor devices are highly integrated, it is generally desirable for an insulated gate field effect transistor as a circuit element for such an integrated circuit to have a gate insulator which is as thin as possible so as to have good characteristics, for example, to increase mutual conductance for rapid operation and lower operating voltage. Reduction of the thickness of a gate insulator, however, will lead to a reduced dielectric breakdown voltage and will give rise to breakdown of the gate insulator during operation as a result of accidental application of high voltage pulses of static electricity.
It is therefore desirable to have a safety device which reacts extremely rapidly so as to safely conduct away excess voltage surges before the insulated gate field effect transistor will suffer from dielectric breakdown, and which as much as possible does not deteriorate the desired performance of the field effect transistor.